The newborn in most mammalian species is dependent upon its mother for a period of days or weeks for immunologic protection against infectious microorganisms or other antigenic assault. By the process of lactation the maternal donor sequesters in her mammary exosecretions antibody molecules and lymphocytes -- both T and B -- for presentation to the suckling. The presence of viable lymphocytes in colostrum not only affords the opportunity for adoptive immunization of the suckling but also presents potential risks for immunologically compromised newborns. Experiments with laboratory rodents indicate that graft-versus-host disease can be procured by suckling and in vitro studies have documented the capability of milk lymphocytes to destroy allogeneic target cells. The potential benefits of cells in milk have not been fully investigated, although we have shown that only a portion of the maternal antigenic reactivities is present in colostrum. It appears that the mammary gland selects antigen-reactive cell clones by a mechanism that, as yet is not well understood with the result that colostral cell-mediated immunity appears to be local in nature much like secretory immunity mediated by IgA antibody. On the basis of these observations, lactation offers a unique opportunity to study local cell-mediated immunity under physiological conditions. Experiments are proposed to further characterize the cells in human colostrum and milk and to determine their potential reactivities. Using animal models and tissue culture techniques, an attempt will be made to determine how colostral immunity is induced, which lymphocytes migrate into colostrum, and how they affect the newborn when ingested. It is expected that our findings will further elucidate the function of lymphocytes in milk and their role in the developing immunocompetence of the newborn.